1. Field of the Invention
The invention generally relates to a structure of an output stage, and more particularly, to a structure of an output stage capable of advancing the driving capacity.
2. Description of Related Art
In recent years, since an operational amplifier has characteristic of high input impedance, high loop gain, low output impedance, low common mode gain and high gain bandwidth, the operational amplifier has been broadly applied in various circuits, for example, in the amplifier circuit for processing the amplification of large signals or in the driving circuit for driving such as capacitive load. In addition, an operational amplifier must have high driving capacity (i.e., high current output) for some large current applications (for example, applied in the driving circuit for driving a display panel).
In an operational amplifier with high driving capacity, the layout of the output stage thereof determines the value of the output current from the operational amplifier, so that the output stage of an operational amplifier occupies a significant position next to circuit design merely and it needs to pay specific attention on the design thereof. If the electrode layouts at the current terminal, ground terminal and output terminal thereof are not appropriate, an electron migration effect may occur, the electrodes may be burn-out, or the equivalent resistance between the electrodes is excessive so as to reduce the momentary maximum current provided by the output stage. When the operational amplifier is packaged as a chip, during the validation check, the output terminal of the operational amplifier would be short-circuit to the power terminal or the ground terminal. At the time, an over current protect (OCP) and an over temperature protect (OTP) are able to protect the operational amplifier for normal operation, but the situation may exist that sometimes the operational amplifier fails to pass validation check.